I don't take away anything from his article. I think he did well by "discovering" the combo.

The problem with bird photography and a full frame body like the 1D X and 5D MKIII is that the tele-converter will be attach almost 100% of the time even using an 800mm lens.

I have the 800mm, 5D MKIII, 1D MKIV and 7D. I've used the 1D X and I found myself on top of the birds to get a decent composition. Of course if you are a big fan of big crops, sure.... A full frame set up will serve you well. Get ready to crop 50% plus for every "keeper". That's why I decided to pass on the 1D X and get a 1D MKIV.

There is absoluty nothing wrong using a 1D MKIV istead as he did for few years. Of couse he has to promote the body. He is paid and associated with Canon.